Estrogens are important for the maintenance of bone mineral density and the prevention of osteoporosis, a disease that affects more than 10 million people in the United States. While we have known about the importance of 17?-estradiol (E2) in bone for decades, we still do not know most of the genes regulated by E2 or how estrogen receptor 1 (ER1) regulates these genes in a tissue specific manner. The goal of this application is to further develop our functional and mechanistic understanding of how estrogen regulates genes to control osteoblast-specific function. We have identified GATA4 as a key pioneer factor for ER1. GATA4 is found at the same enhancers as ER1 in osteoblast cell lines and is required for ER1 recruitment to these enhancers. We hypothesize that GATA4 interacts with a distinct complex of proteins at ER1 binding sites to control osteoblast differentiation. Aim 1 will determine how GATA4 controls tissue-specific transcription induced by estrogen. Finally Aim 3 will extend these studies to analyze GATA4 in vivo by creating an osteoblast-specific knockout of GATA4. Aim 2 will determine the mechanism by which GATA4 functions in osteoblast differentiation and mineralization. Together these experiments will for the first time describe how novel osteoblast transcription factor (GATA4) regulates estrogen biology in bone. PUBLIC HEALTH RELEVANCE: Osteoporosis is a significant public health concern that affects over 10 million people in the United States and, with the aging population, this number is likely to increase in the next few decades. Estrogens are important in the development of bone and maintenance of bone mineral density in mice and humans. The goal of this application is to further develop our functional and mechanistic understanding of how estrogen regulates bone cells by its interaction with another transcription factor GATA4.